A thermal sublimation transfer recording process is known, in which printing paper having a dye-receiving layer is attached to an ink ribbon having an ink layer comprising a subliming or hot-diffusing dye and the ink layer of the ink ribbon is heated with a thermal head or the like in accordance with image information whereby the dye is transferred from the ink layer to the dye-receiving layer of the printing paper to form an image on the paper. According to this process, it is possible to form full-color images with continuous gradations. Therefore, the process has been considered useful for forming hard copies from video images.
FIG. 1 is a cross-sectional view of an ordinary printing paper 1 which is used for thermal sublimation transfer recording. As shown in this, the printing paper 1 has a laminate structure comprising a sheet substrate 2 and a dye-receiving layer 3. The dye-receiving layer 3 receives the dye that has been transferred from an ink ribbon by thermal transfer recording and keeps the image of the dye. The dye-receiving layer 3 of this type comprises a dyeable resin such as polyesters, cellulose esters, polycarbonates, polyvinyl chlorides, etc.
Recently, printing paper such as that shown in FIG. 1 is required to have the following properties in order that it may be applied to high-speed printers.
(i) It has high dye-fixability and can form a glossy and sharp image having a high density.
(ii) The storage stability of the image formed thereon is good. Precisely, (a) the image has good fingerprint resistance and sebum resistance. Concretely, when the image is contacted with a part of a human body such as hands, fingers, etc., the dye forming the image is neither aggregated nor faded. (b) The image has good plasticizer resistance. Concretely, when the image is contacted with a plastic eraser containing a plasticizer or its wastes, the dye forming the image is neither aggregated nor faded. (c) The image has high light resistance in order that it is neither faded nor discolored when exposed to light. (d) The image has dark fading resistance.
In order to satisfy these requirements, various proposals have been made for the constitution of printing paper. For instance, U.S. Pat. No. 4,731,355 has disclosed the use of a butyral resin as the essential component in the dye-receiving layer of printing paper. U.S. Pat. Nos. 5,187,144 and 5,332,712 have disclosed the use of a polyvinyl acetal resin as the essential component in the dye-receiving layer of printing paper.
However, the conventional printing paper where the dye-receiving layer is made of a dyeable resin such as polyesters, etc. had problems in that the light resistance, the dark fading resistance, the sebum resistance and the plasticizer resistance of the image formed are not sufficient and therefore the storability of the image is poor. Even the printing paper comprising, as the essential component in the dye-receiving layer, a butyral resin or a polyvinyl acetal resin does not still have satisfactory storability, and the improvement in this respect has been desired. In particular, indaniline dyes are useful as cyanine dyes having high transfer sensitivity, but the light resistance of images of such indaniline dyes was insufficient.
In order to solve the problem of the storability of the image formed, a storability-improving agent such as an UV absorbent, an antioxidant, etc. is added to the dye-receiving layer, which, however, does not attain a sufficient result. In order to improve the sebum resistance and the plasticizer resistance of the image formed, a cover film is laminated over the printing paper having an image formed thereon, which, however, is problematic in that it needs the laminating step in addition to the image-forming step in the conventional thermal transfer recording process. In addition, the outward appearance and the thickness of the printing paper laminated with the cover film are often problematic.